The lids or covers of electronic microdevices such as hearing aids are usually attached at one end to a hinge around whose pin they can pivot, allowing them to be opened and closed so as to cover an opening in the housing of the electronic microdevice.
There is a risk especially in the case of new-generation in-ear hearing aids to the effect that the cover plate of the housing or of the battery compartment may not close tightly. The hinge section is particularly difficult to seal tightly, and the acoustic seal as well cannot be optimized.
A contributor to the problem is the fact that the technology employed in producing housings for in-ear hearing aids does not offer the precision needed to assure a seal that is 100% tight.